We are seeking to identify the substances that serve as physiologic regulators of liver growth, and to define the mechanisms through which they operate. Previous studies, carried out largely in the whole animal, have suggested possible involvement of insulin, glucagon and epidermal growth factor (EGF). We have turned to primary monolayer cultures of adult rat hepatocytes for further assessment of these substances; as in vivo, EGF stimulates DNA synthesis, and this effect is enhanced by the pancreatic hormones. Rat serum is generally even more effective than the EGF-hormone mixtures, and is several fold more stimulatory than bovine, equine, murine or human sera. About half of the activity of rat serum derives from the blood platelets, and we are attempting to characterize and isolate the active principle which differs considerably from human PDGF. Contrary to expectation from in vivo studies, we so far find little or no difference in DNA biosynthetic stimulation by serum from partially hepatectomized compared to normal rats, and we are searching for the cause of this discrepancy. We are also employing the culture system to explore a number of other putative growth stimulatory substances and crude biological materials.